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176 CHAPTER SEVEN
Spy-hopping
Some whales have an unusual practice of rising out of the water to see what is going on
above the water line (see Figure 7-4). It gives them some visibility they might not have
while underwater. Interfaces also spy-hop to detect network activity. The interface only
looks at the network periodically so energy use is minimized when no activity exists.

Spy-hopping networks Peripheral network interface chips are available that can
monitor the network periodically to see if there are messages. Other interface
chips are capable of waking the system up from a power-saving slumber when a
message destined only for the robot appears. The rest of the time, the interface cir-
cuitry is turned off to save energy.
Spy-hopping energy detection It takes more circuitry to detect specific data
patterns than it takes simply to detect data activity. Some communication inter-
faces will sleep until they detect a sufficient amount of energy at the communi-
cation input. This works best in a communication link where there is little traffic
that is not meant for the receiver (like a narrowband radio frequency [RF] link).
This type of wakeup does not work well in networks where all interfaces share the
same physical link, differentiated only by addressing.
Spy-hopping time coordination If both ends of a communication link agree in
advance to limit communication to distinct time windows, neither side of the link

FIGURE 7-4 A whale spy-hopping